1. Field of the Invention
The present invention is directed generally toward the testing of thin film solar modules to detect malfunctions and, more specifically, toward a method and apparatus for testing a plurality of individual photovoltaic cells electrically coupled together in series to localize any of the individually tested photovoltaic cells that are not performing as desired.
2. Description of Related Art
Traditionally, photovoltaic (“PV”) cells referred to as crystalline silicon PV cells included a substrate formed from crystalline silicon to act as the light-absorbing semiconductor. Light energy from the sun is absorbed within the silicon and converted into electric energy. Single-crystal silicon wafers sliced from an ingot of single-crystal silicon are most often used because they offer favorable conversion efficiencies, and thus produce a desirable output of electrical energy.
But crystalline silicon PV cells have significant drawbacks, the most notable of which include cost, size and durability. The single-crystal silicon ingot is produced by a lengthy, costly and heat-and-pressure-sensitive process. Moreover, single-crystal silicon is also the semi-conducting material from most conventional microprocessors are formed. This creates a significant demand for a limited supply of wafers formed from single-crystal silicon, and in turn, high costs associated with such wafers.
The process of producing single-crystal silicon wafers also limits the maximum diameter of the wafers on which individual PV cells are to be formed. Typically, a single-crystal silicon ingot is drawn from a hot silicon melt. Single-crystal silicon grows from a center axis of the ingot radially outward to produce an ever increasing diameter. However, the longer the single-crystal silicon is permitted to grow the more likely it is that imperfections will be introduced into the silicon crystal lattice from conditions such as temperature and/or pressure fluctuations, the speed at which the ingot is drawn from the melt, impurities in the melt. Thus, to create an array of PV cells large enough to produce the desired output of electric energy many single crystal silicon wafers will be needed. Due to the cost considerations mentioned above this will often make crystalline silicon PV cells economically impractical for commercial applications.
Further, crystalline silicon PV cells are typically fragile and sensitive to environmental conditions such as moisture. This requires sealing the PV cells of an array within a rugged enclosure, adding additional costs to such arrays of crystalline PV cells.
Considering these drawbacks, advances in PV cell technology have been focused on thin film PV cells. Unlike the crystalline PV cells, thin film PV cells include a light-absorbing semiconductor formed from materials with less stringent processing requirements such as amorphous silicon, or other polycrystalline materials. Further, thin film PV cells can be on the order of one micron (i.e., one-millionth of a meter) thick, requiring less of the light-absorbing material than their crystalline counterparts. Thin film PV cells can be realized on flexible substrates and have a high durability. And because a single-crystal semiconductor material is not required, the light-absorbing semiconductor of thin film PV cells is conducive to large area deposition techniques. Thin film PV cells can form arrays on large substrates having surface areas of 1 m2 to more than 4 m2.
Although thin film PV cells include more cost effective and readily-available materials, they do not produce as much electric energy as crystalline silicon PV cells, on average. Because of the lower average output of individual thin film PV cells compared to crystalline silicon PV cells, the arrays of thin film PV cells are larger, including many more individual thin film PV cells than crystalline silicon PV cells required to produce a comparable output voltage. When such an array of thin film PV cells does not perform as expected it is often the result of an imperfection in one or more of the individual thin film PV cells. But due to the large number of such cells in the array, however, locating the offending cells is a daunting and time-consuming task.
Accordingly, there is a need in the art for a method and apparatus for testing thin film PV cells collectively forming a thin-film solar module. The method and apparatus can identify thin film PV cells within the module that are not generating a desired output and identify a location of those thin film PV cells within the module.